1. Field of the Invention
The present invention relates to a diffractive optical element used for an optical system or the like and a method for manufacturing the diffractive optical element.
2. Description of the Related Art
As one of the methods for correcting chromatic aberration in an optical system, a method has been known in which two lenses composed of glass materials that differ in dispersion properties are combined. Meanwhile, another method has been known in which a diffractive optical element having a diffraction effect is provided on a lens surface to thereby reduce chromatic aberration. This method uses a physical phenomenon where the refractive and diffractive surfaces in an optical system exhibit chromatic aberration outputs in opposite directions with respect to a light ray having a given reference wavelength.
Furthermore, in order to adjust refractive indices and Abbe numbers of diffractive optical elements, U.S. Pat. No. 6,759,471 (Patent Document 1) discloses a composite material in which fine particles composed of a transparent conductive metal oxide, such as ITO, ATO, SnO2, or ZnO, are mixed/dispersed in a UV curable binder resin. Furthermore, Patent Document 1 also discloses a laminated diffractive optical element formed by laminating two resin layers. In an optical system having a chromatic aberration correction effect, such a laminated diffractive optical element can greatly reduce diffraction efficiency in the vicinity of a designed order in the wavelength region to be used.
In recent years, when a diffractive optical element is used as a camera lens, nano-level shape accuracy of a diffraction grating may be required. However, in the case where a photo-curable resin is used, since the resin starts to react from the points irradiated with ultraviolet light or the like, a difference in density occurs in the cured resin due to the difference in the curing rate, resulting in a non-uniform refractive index distribution.